1. Field of the Invention
The present invention relates to a membrane for delivering a drug which membrane is capable of delivering a drug in a suitable manner and, therefore, of delivering the drug in a pattern suitable for the characteristic properties of the drug.
2. Prior Art
Drugs are usually administrated in the form of an oral preparation, external preparation, suppository, injection or inhalant depending on the symptoms of the patient, and the pharmacological effect and pharmacokinetical properties of the drug. However, the oral preparations have problems such as irritation of the gastrointestinal system and reduction in the bioavailability due to the so-called first-pass effect in the liver. When a drug is used as an external preparation, problems arise in that the absorption of the drug may be difficult and also that the control of the dose thereof may also be difficult. When a drug is used as a suppository, the absorption of the drug varies depending on the quantity of the fecal matter remaining in the rectum. When a drug is used in an injection, the patient experiences a sharp pain and the continuous use thereof over a long period of time is difficult.
Under these circumstances, intensive studies are being made for the purpose of developing drug delivery systems (DDS). Practically applied the drug delivery systems include external preparations of percutaneous absorption type such as nitroglycerin and nitrous esters for the treatment of angina and scopolamine for the prevention of motion sickness are generally used.
However, these preparations are made only for the purpose of slowly delivering the drug and are essentially different from a DDS capable of controlling the quantity of the drug to be delivered depending on the symptoms.
Recently it was reported that when a membrane comprising a dialkyldimethylammonium bromide deposited on a polymer membrane is used, the diffusion coefficient of a substance in the membrane varies depending on temperature change. These facts are described in, for example, J. American Chem. Soc., 105, No. 15, 4855-4859 (1983) and Chemistry Letters, 645-648 (1979).
However, such a membrane does not respond to only a slight temperature change and the change of the diffusion coefficient in such a case is not so significant. Substances whose diffusion can be controlled by the temperature changes are limited to small ions such as sodium and chlorine ions. Thus, the diffusion of large molecules having pharmacological effects cannot be controlled yet. In addition, a dialkyldimethylammonium bromide deposited on the polymer membrane is ionic and, therefore, it is unsuitable for controlling the delivery of molecules of various ionic drugs.